1. Field of the Invention
The present invention relates to a phase-locked oscillator, and more particularly to a circuit for indicating when the tuning voltage of a phase-locked voltage controlled oscillator (VCO) is out of range and causes an "out-of-lock" condition.
2. Description of the Prior Art
A phase-locked oscillator often must be brought close to its correct frequency before phase lock occurs. One method for doing this is to employ an additional feedback loop to cause the tuning voltage to oscillate. When phase lock occurs the feedback of the phase-locked loop overcomes the added loop and stops the oscillation. Sometimes the added loop includes a Schmidt trigger that changes state at the tuning voltage sweep limits. The Schmidt trigger is connected such that the change in state causes the sweep voltage to reverse direction, producing a saw-tooth wave. The Schmidt trigger has the advantage of eliminating the influence of the added loop during steady-stage operation. The sweeping voltage is detected to indicate an out-of-lock-condition.
However, the out-of-lock indicator in such a circuit fails if the tuning voltage does not sweep, even though the circuit is not phase-locked. This occurs if the loop amplifier fails or if the voltage drift in some component, particularly the phase detector, is sufficient to prevent the sweep voltage from reaching a limit. Ordinarily, the Schmidt trigger's square-wave output is converted to approximately a saw-tooth by low passing, but if the sweeping tuning voltage does not reach a limit, the sweep voltage will be an exponential which reaches steady-state before attaining the limit. Because of high gain in the loop amplifier, slightly more drift in the same direction causes the tuning voltage to go beyond the other sweep limit. This problem cannot be solved by arbitrarily increasing the amplitude of the sweeping voltage because an excessively large sweep voltage can overcome the feedback of the phase-locked loop and prevent locking.